Diesel fuel is a refined petroleum product which is burned in the engines powering many of the world's trains, ships, and large trucks. Petroleum is a non-renewable resource of finite supply. Acute shortages and dramatic price increases in petroleum and the refined products derived from petroleum have been suffered by industrialized countries during the past quarter-century. Furthermore, diesel engines which run on petroleum based diesel emit relatively high levels of certain pollutants, especially particulates. Accordingly, extensive research effort is now being directed toward replacing some or all petroleum-based diesel fuel with a cleaner-burning fuel derived from a renewable source such as farm crops, waste animal fats, and other suitable waste materials.
In an effort to partially replace dependence on petroleum based diesel, vegetable oils have been directly added to diesel fuel. These vegetable oils are composed mainly of triglycerides, and often contain small amounts (typically between 1 and 10% by weight) of free fatty acids. Some vegetable oils may also contain small amounts (typically less than a few percent by weight) of mono- and di-glycerides.
Triglycerides are esters of glycerol, CH2(OH)CH(OH)CH2(OH), and three fatty acids. Fatty acids are, in turn, aliphatic compounds containing 4 to 24 carbon atoms and having a terminal carboxyl group. Diglycerides are esters of glycerol and two fatty acids, and monoglycerides are esters of glycerol and one fatty acid. Naturally occurring fatty acids, with only minor exceptions, have an even number of carbon atoms and, if any unsaturation is present, the first double bond is generally located between the ninth and tenth carbon atoms. The characteristics of the triglyceride are influenced by the nature of their fatty acid residues.
The production of alkyl esters of glycerides by transesterification is a known process. However, transesterification suffers in that the reaction generally requires the addition of an acid or base catalyst which must be neutralized after the reaction thereby generating salts and soaps. In addition, while transesterification results in the separation of fatty acid esters from triglycerides, it also results in the production of glycerin, which must then be separated from the fatty acid esters, glycerin, excess alcohol, salts, and soaps.
The production of alkyl esters of fatty acids by acid catalyzed esterification is also known. However, the use a strong acid, such as sulfuric acid, typically leads to higher sulfur content in the resulting ester as the acid reacts with the double bonds in the fatty acid chains. In addition, conversion of the esterification reaction is limited by equilibrium constraints such that either excessive time and equipment size are required or less conversion needs to be accepted.
In an effort to overcome some of the problems associated with transesterification, several attempts have been made to employ esterification between fatty acids and alcohols. In these processes, fatty acids are prepared from triglycerides by hydrolysis, followed by catalyzed esterification of the fatty acids with an alcohol, preferably methanol. While this practice is practiced in the production of fatty alcohols and fatty acid esters, as described in U.S. Pat. No. 5,536,856 (Harrison et al), it has not been practiced in the production of Renewable Diesel.